Social Post Stamps

These are fun and good-looking options that have the contoured edges of postage stamps. They're easy to read and users shouldn't have any trouble deciphering them.

http://www.marcofolio.net/icon/social_post_stamps_free_icon_set.html

Function Free Social Media Icon Bonus Pack

These are very straightforward. They have some cool design aspects to them but they're most notable for the fact that they're very easy to read.

http://wefunction.com/2009/05/free-social-icons-app-icons/

Heart My Website

These are heart-shaped, just as the name implies. They're fun for personal sites but probably a bit lighthearted for a professional site, unless it actually covers a subject matter where these shapes are appropriate.

Social Media Mini Icon Pack

As the name implies, these are very small. They come for all the major social networking sites. These would work well for blogs that have a lot of screen space used up by content and that want to put icons up but don't want them to crowd out the page.

http://www.komodomedia.com/blog/2008/12/social-media-mini-iconpack/

You can, of course, always get individual icons right from the site you're linking to!